Justification:
Successful conservation has increased the population of this species, but it remains extremely small and the species consequently qualifies as Endangered. If the population continues to grow, which recent figures suggest it has, the species will warrant downlisting to Vulnerable in due course.

Pterodroma cahow once bred abundantly throughout Bermuda (to UK). It was thought extinct for almost three centuries, until reported (with specimens) during the first half of the 20th century. In 1951, 18 pairs were rediscovered breeding on suboptimal rocky islets (total area 1 ha) in Castle Harbour. Intensive management has resulted in slow but steady increases, and the population was estimated at 250 birds in 2005 (J. Madeiros in litt. 2005), with 70 pairs fledging a record 40 young in 2003 (Madeiros 2003), and 71 pairs fledging 35 young in 2005 (J. Madeiros in litt. 2005). More recently, 40 young fledged in 2008 from 85 established active nest sites and at least 35 chicks hatched in 2009 (Madeiros unpublished data). By 2011, the population reached 98 nesting pairs (Madeiros 2011). From 40 chicks on all islands in 2008, 21 chicks were translocated in May to Nonsuch, forming the last year of the translocation project (Madeiros 2008). Fourteen individuals fledging from Nonsuch Island after translocation in 2005 and 2006 were observed in 2009 returning to the island and entering artificial burrows. One chick was born on the island in 2009 (Madeiros unpublished data). By 2011, 22 translocated birds have returned to Nonsuch, and 8 more translocated birds returned to the original nesting islets. Non-translocated birds have also been recorded on Nonsuch indicating a sufficient nucleus of translocated birds to attract non-translocated individuals into the colony (Madeiros 2011). In the non-breeding season, birds probably move north into the Atlantic, following the warm waters on the western edges of the Gulf Stream (Wingate 1997). There are confirmed records off the coast of North Carolina, Virginia, Massachusetts, USA (D. S. Lee in litt. 1998, Wingate et al. 1998, Nisbet et al. 2013, Maderios et al. 2014, Adams et al. 2014, Sattelmeyer et al. 2015), and one bird was captured in the Azores in November 2002 and recaptured in November 2003 and December 2006 (Bried 2003, J. Bried in litt. 2010).

The population is estimated to number 250 individuals, including 71 breeding pairs and thus a minimum 142 mature individuals (J. Madeiros in litt. 2005).

Trend Justification: The population has increased from 18 pairs in 1951 to 71 pairs in 2005. This is equivalent to an increase of well over 79% in three generations, given the species's long lifespan. Records of 40 chicks fledged in 2008 and 35 chicks hatched in 2009 suggest the population continues to increase.

It formerly nested in soil burrows, but such habitat is not available on current breeding islands and it now nests in suboptimal, natural erosion limestone crevices and artificial burrows. The breeding season is January-June, and breeding success has increased from less than 5% per year in the 1950s to more than 25% per year in the 1990s (Wingate 1997). Ringing recoveries have shown that birds first return to breed four years after fledging (Madeiros in litt. 2006). The breeding grounds are not visited by birds between mid-June and mid-October (Wingate 1997), with adults returning from mid-October (Madeiros 2008). Results from geolocator tags showed that individuals can cover in excess of 5,000 km during a foraging trip, following different courses from Bermuda but generally all foraging over the Gulf Stream (Madeiros unpublished data). During the non-breeding season they have been recorded in the Gulf Stream, north to the Bay of Fundy, into the Gulf of St. Lawrence and over the Grand Banks, with one individual recorded 201-241 km off southwest Ireland (Madeiros 2009).

The drastic population decline is attributed to habitat loss, exploitation and predation. Its recovery has been hampered by competition from White-tailed Tropicbird Phaethon lepturus for nest-sites and subadult predation from a single Snowy Owl Bubo scandiacus (the first ever recorded in Bermuda) on Nonsuch Island, which was eradicated, having eaten 5% of the population (Dickinson 2007). Light pollution from a nearby airport and NASA tracking station adversely affects nocturnal aerial courtship (Wingate et al. 1998). The threat of sea-level rise and increased storm activity appears real, with five or more major floods affecting burrows in the 1990s, after 25 years without significant problems (Wingate et al. 1998). Category three hurricane Fabian in 2003 overwashed three of the four breeding islets, damaging or destroying a significant number of nest burrows (J. Madeiros in litt. 2005). Increasing egg failure may be attributable to contaminants (Wingate 1997). Rats also swam to one breeding island in April 2005, but were successfully eradicated within two weeks without loss to the Cahows (J. Madeiros in litt. 2005). Unfortunately this pattern appeared to be repeated in March 2008, with four chicks killed on one of the nesting islets. Immediate baiting produced a dead black rat Rattus rattus. However, as the islands were all baited at the beginning of the nesting season, this incident pointed out the need for constant vigilance of reintroduction and a requirement to provide fresh bait on the islands throughout the nesting season (Madeiros 2008).

Conservation Actions UnderwayCMS Appendix I. Since 1961, there has been management of nesting-grounds, including the creation of artificial burrows, periodic removal of rats and the elimination of nest-site competition from P. lepturus (by installing baffles over burrow entrances). As part of the Bermuda Conservation Programme, potential breeding islands (e.g. Nonsuch) have been reforested with native flora in an attempt to attract nesting petrels (Wingate et al. 1998). The Castle Harbour islands are a National Park and Nature Reserve (J. Madeiros in litt. 2005). In 2004 and 2005 efforts were made to attract adult Cahows, displaced from low-lying nest burrows destroyed by hurricane Fabian, on the main breeding islet to a new artificial burrow complex built on a more elevated section of the islet. Using a combination of a sound attraction system set up among the new nests and physical translocation of adult Cahows from the destroyed sites, three pairs occupied burrows in the new complex by March 2005 (J. Madeiros in litt. 2005). A project is now underway to establish a new nesting colony on the Nonsuch Island Living Museum, which is much larger and contains more suitable breeding habitat than the present suboptimal breeding islets. The project involves physical translocation of chicks from the present breeding islets to a new complex of artificial burrows on Nonsuch, so that they will imprint on the new site and return when mature to establish their nests at the new location. In 2004, the trial year of the project took place with 14 chicks moved to Nonsuch, where they were fed and monitored every other day until departure, with all fledging successfully. In 2005, 21 chicks were translocated, with all again fledging successfully by mid-June. This project was scheduled to continue for three more years, with a target of 90 to 100 chicks in total being translocated over a five-year period (J. Madeiros in litt. 2005). In 2009, the first adult-fed Cahow for 400 years hatched on Nonsuch Island (Dobson 2009). So far, 79 relocated chicks have fledged on Nonsuch, with only one fatality (Dickinson 2007). A Sound Attraction System was also set up in 2007 to help encourage birds to stay and prospect on Nonsuch, and overcome any tendency for young birds to be attracted back to the activity at the original nesting islets (Madeiros 2008). A total of 171 fledglings was ringed between 2002-2007, with 31 confirmed having returned (including 4 recaptured on Nonsuch Island), representing cohorts from four nesting seasons, 2002 to 2005 (Madeiros 2008).

Conservation Actions ProposedMaintain all management activities at current nesting-grounds. Investigate whether contaminants are increasing egg failure (Wingate 1997). Continue to manage the new breeding colony on Nonsuch Island (Wingate 1997, Madeiros 2003, J. Madeiros in litt. 2005). Investigate the pelagic and foraging range of the species using new data logger technology (J. Madeiros in litt. 2005).